Overseas Road Note 3 (2nd Edition)

A guide to surface dressing in tropical andsub-tropical countries

ORN 3 (2 nd Edition)

First Published 2000ISSN 0951-8797Copyright TRL Limited 2000.

Subsector: Transport

Theme: T2

Project title: A guide to surface dressing in tropical and sub-tropical countries

Project reference: D251

This document is an output from a project funded by the UKDepartment for International Development (DFID) for thebenefit of developing countries. The views expressed are notnecessarily those of the DFID.

IRL is committed to optimising energy efficiency, reducingwaste and promoting recycling and re-use. In support of theseenvironmental goals, this report has been printed on recycledpaper, comprising 100% post-consumer waste, manufacturedusing a TCF (totally chlorine free) process.

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ACKNOWLEDGEMENTS

This Overseas Road Note was produced in the InternationalDivision of TRL (Director Mr S W Colwill) on behalf of theDepartment for International Development. The research hasbeen carried out with the active collaboration of highwayauthorities in many countries and their help and cooperationhas been essential to the success of the project. The TRLproject team responsible for this Road Note were Mr H RSmith (Project Officer), Dr J Rolt and Mr W G Ford. Thehelpful comments of the World Bank are gratefullyacknowledged. Where necessary, use has been made ofwork published by other research and road authorities.

OVERSEAS ROAD NOTES

Overseas Road Notes are prepared principally for road andtransport authorities in countries receiving technicalassistance from the British Government. A limited numberof copies is available to other organisations and toindividuals with an interest in roads overseas, and may beobtained from:

International Development Advisory and Information UnitTRL LimitedCrowthome, Berkshire, RG45 6AUUnited Kingdom

Limited extracts from the text may be reproduced providedthe source is acknowledged. For more extensivereproduction, please write to the address given above.

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CONTENTS

Page

Preface 1

1 Introduction 3

2 Types of surface dressing 3Single surface dressing 3

Double surface dressing 3Triple surface dressing 4Racked-in surface dressing 4

Other types of surface dressing 5

3 Chippings for surface dressings 5

4 Bitumens 6Prime coats 6

Bitumens for surface dressings 7Penetration grade bitumens 7Bitumen emulsion 7

Cutback bitumens 8Polymer modified bitumens 9Adhesion agents 10

5 Design 10

Existing site conditions 10Selecting the binder 12

Choice of binder and timing of construction work 13

Designing the surface dressing 13Basis for the design method 13Determining the average least dimension of chippings 14

Determining the overall weighting factor 15Determining the basic bitumen spray 15Spray rate adjustment factors 15

Adjusting rates of spray for maximum durability 17Surface dressing design for low volume roads 17Spread rate of chippings 17

6 Plant and equipment

Methods of distributing binder 18Constant volume distributors 18Constant pressure distributors 19

Principal components of binder distributors 19Spray bars and spray jets 19Binder pumps and air pumps 20

Tanks and burners 20

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Page

Distributor speed control and calibration 20Chip spreaders 21

Rollers and other equipment 21

7 The surface dressing process 22

Planning 22The surface dressing operation 23

After-care 23

8 Other surface treatments 24

Slurry seals 24Otta seal 25

Sand seals 25Synthetic aggregate and resin treatments 25Applications of light bitument sprays 25

Fog sprays 26Enrichment sprays 26

9 References 26

Appendix A: Requirements for grading and particle shape(Reproduced from BS 63: Part 2: 1987) 28

Appendix B: Recommended polished stone values of chippingsfor roads in Britain 29

Appendix C: The immersion tray test for determining theconcentration of adhesion agent required 30

Appendix D: The probe penetration for test for measuringroad surface hardness 31

Appendix E: Example of a surface dressing design 32

Appendix F: Tests for uniformity of transverse distributionof binder (depot tray tests) (reproduced fromBritish Standard 1707:1989) 33

Appendix G: Bitumen distributor: rate of spread/speedcalibration 35

Appendix H: Miscellaneous equipment required for asurface dressing unit 35

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Preface

This Road Note is the 2nd edition of Overseas RoadNote 3 'A guide to surface dressing in tropical andsub-tropical countries'

The revisions include descriptions of a wider range oftypes of surface dressing, current materials

specifications and more detail on the use of bitumenemulsions. An equation has been introduced to allowthe design spray rate to be calculated as an alternative

to reading from a nomograph. A simplifiedpresentation of spray rate adjustment factors, relatedto different site conditions, has also been provided.

The durability of surface dressings, particularly forlow volume roads, is discussed and additional spray

rate adjustment factors have been suggested for theseroads. The use of Otta seals has also been suggestedfor low volume roads.

It is important that this Road Note, is not regarded asa specification. However, the advice contained in this

Road Note together with local experience of relevantmaterials and surface dressing performance should beof value to those drawing up specifications.

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1 Introduction

1.1 Surface dressing is a simple, highly effective andinexpensive road surface treatment if adequate care is taken inthe planning and execution of the work. The process is usedthroughout the world for surfacing both medium and lightly-trafficked roads, and also as a maintenance treatment for roadsof all kinds.

1.2 Surface dressing comprises a thin film of binder,generally bitumen or tar, which is sprayed onto the roadsurface and then covered with a layer of stone chippings. Thethin film of binder acts as a waterproofing seal preventing theentry of surface water into the road structure. The stonechippings protect this film of binder from damage by vehicletyres, and form a durable, skid-resistant and dust-free wearingsurface. In some circumstances the process may be repeated toprovide double or triple layers of chippings.

1.3 Surface dressing is a very effective maintenancetechnique which is capable of greatly extending the life of astructurally sound road pavement if the process is undertakenat the optimum time. Under certain circumstances surfacedressing may also retard the rate of failure of a structurallyinadequate road pavement by preventing the ingress of waterand thus preserving the inherent strength of the pavementlayers and the subgrade.

1.4 In addition to its maintenance role. surface dressingcan provide an effective and economical running surface fornewly constructed road pavements. Existing roads withbituminous surfacings, carrying in excess of 1000vehicles/lane/day, have been successfully surfaced withmultiple surface dressings. For sealing new roadbases trafficflows of up to 500 vehicles/lane/day are more appropriate,although this can be higher if the roadbase is very stable or ifa triple seal is used. A correctly designed and constructedsurface dressing should last at least 5 years before resealingwith another surface dressing becomes necessary. If trafficgrowth over a period of several years necessitates a moresubstantial surfacing or increased pavement thickness, abituminous overlay can be laid over the original surfacedressing when the need arises.

1.5 The success of a surface dressing depends primarily onthe adhesion of the chippings to the road surface, hence boththe chippings and the road surface must be clean and freefrom dust during the surface dressing process. Inappropriatespecifications, poor materials, and bad workmanship, can alsodrastically reduce the service life of a surface dressing.

1.6 This Road Note is a general guide to the design andconstruction of surface dressings in tropical and sub-tropicalenvironments and draws attention to some of the morecommon mistakes that are made. It

provides a framework on which the engineer can base morespecific decisions made to suit particular local conditionsthereby producing cost effective results. It also contains briefdescriptions of certain other types of surface treatment.

2 Types of surface dressing

2.1 Surface dressings can be constructed in a number ofways to suit site conditions. The common types of dressingare illustrated in Figure 1.

Single surface dressing2.2 When applied as a maintenance operation to anexisting bituminous road surface a single surface dressing canfulfil the functions required of a maintenance re-seal, namelywaterproofing the road surface, arresting deterioration, andrestoring skid resistance. A single surface dressing would notnormally be used on a new roadbase because of the risk thatthe film of bitumen will not give complete coverage. It is alsoparticularly important to minimise the need for futuremaintenance and a double dressing should be considerablymore durable than a single dressing. However, a 'racked-in'dressing (see paragraph 2.7) may be suitable for use on a newroadbase which has a tightly knit surface because of theheavier applications of binder which is used with this type ofsingle dressing.

Double surface dressing2.3 Double surface dressings are robust and should be usedwhen:

• A new roadbase is surface dressed.

• Extra 'cover' is required on an existing bituminous roadsurface because of its condition (e.g. when the surface isslightly cracked or patched).

• There is a requirement to maximise durability andminimise the frequency of maintenance and resealingoperations.

2.4 The quality of a double surface dressing will begreatly enhanced if traffic is allowed to run on the firstdressing for a minimum period of 2-3 weeks (and preferablylonger) before the second dressing is applied. This allows thechippings of the first dressing to adopt a stable interlockingmosaic which provides a firm foundation for the seconddressing. However, traffic and animals may causecontamination of the surface with mud or soil during thisperiod and this must be thoroughly swept off before thesecond dressing is applied. Such cleaning is sometimesdifficult to achieve and the early application of the secondseal to prevent such contamination may give a better result.

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Figure 1 Type of surface dressings

2.5 Sand may sometimes be used as an alternative tochippings for the second dressing. Although it cannotcontribute to the overall thickness of the surfacing, thecombination of binder and sand provides a useful groutingmedium for the chippings of the first seal and helps to holdthem in place more firmly when they are poorly shaped. Aslurry seal may also be used for the same purpose (seeparagraph 8.2).

Triple surface dressings2.6 A triple surface dressing (not illustrated in Figure 1)may be used to advantage where a new road is expected tocarry high traffic volumes from the outset. The applicationof a small chipping in the

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third seal will reduce noise generated by traffic and theadditional binder will ensure a longer maintenance-freeservice life.

Racked-in surface dressing2.7 This system is recommended for use where traffic isparticularly heavy or fast (TRL, 1996). A heavy singleapplication of binder is made and a layer of large chippings isspread to give approximately 90 per cent coverage. This isfollowed immediately by the application of smaller chippingswhich should ‘lock-in' the larger aggregate and form a stablemosaic. The amount of bitumen used is more than would beused with a single seal but less than for

a double seal. The main advantages of the racked-in surfacedressing are:

• Less risk of dislodged large chippings.

• Early stability through good mechanical interlock.

• Good surface texture.

Other types of surface dressing2.8 'Sandwich' surface dressings are principally used onexisting binder rich surfaces and sometimes on gradients toreduce the tendency for the binder to flow down the slope.

2.9 'Pad coats' are used where the hardness of the existingroad surface allows very little embedment of the first layer ofchippings, such as on a newly constructed cement stabilisedroadbase or a dense crushed rock base. A first layer ofnominal 6mm chippings will adhere well to the hard surfaceand will provide a 'key' for larger l0mm or l4mm chippings inthe second layer of the dressing.

3 Chippings for surface dressings

3.1 The selection of chipping sizes is based on the volumeof commercial vehicles having unladen weights of more than1.5 tonnes and the hardness of the existing pavement. Ideally,chippings used for surface dressing should be single sized,cubical in shape, clean and free from dust, strong, durable, andnot susceptible to polishing under the action of traffic. Inpractice the chippings available usually fall short of this idealbut it is recommended that chippings used for surface dressingshould comply with the requirements of BS 63: Part 2 (1987)for the nominal size of chipping selected by the engineer. Inthis standard, some control of shape is ensured by the limitsset for the flakiness index for each nominal size (except6mm). Part of BS 63 is reproduced in Appendix A.

3.2 Samples of the chippings should be tested for grading,flakiness index, aggregate crushing value and, whenappropriate, the polished stone value and aggregate abrasionvalue. Sampling and testing should be in accordance with themethods described in British Standard BS 812 (1985,1989a,1989b 1990a, 1990b).

3.3 Specifications for maximum aggregate crushing value(ACV) for surface dressing chippings typically lie in the range20 to 35. For lightly trafficked roads the higher value is likelyto be adequate but on more heavily trafficked roads amaximum ACV of 20 is recommended.

3.4 The polished stone value (PSV) of the chippings isimportant if the primary purpose of the

surface dressing is to restore or enhance the skid resistance ofthe road surface. The PSV required in a particular situation isrelated to the nature of the road site and the speed andintensity of the traffic (Salt and Szatkowski, 1973). Theresistance to skidding is also dependent upon the macrotexture of the surface which, in turn, is affected by thedurability of the exposed aggregate. This property ismeasured by the aggregate abrasion value (AAV). AppendixB gives recommended values of PSV and AAV for variousroad and traffic conditions in Britain and provides anindication of the required aggregate properties.

3.5 The nominal sizes of chippings normally used fo rsurface dressing are 6, 10, 14 and 20 mm. Flaky chippings arethose with a thickness (smallest dimension) less than 0.6 oftheir nominal size. The proportion of flaky chippings clearlyaffects the average thickness of a single layer of thechippings, and it is for this reason that Jackson (1963)introduced the concept of the 'average least dimension' (ALD)of chippings.

3.6 In effect, the ALD is the average thickness of a singlelayer of chippings when they have bedded down into theirfinal interlocked positions. The amount of binder required toretain a layer of chippings is thus related to the ALD of thechippings rather than to their nominal size. This is discussedfurther in Section 5 where guidance is given on the selectionof the appropriate nominal size of chipping and the effect offlakiness on surface dressing design.

3.7 The most critical period for a surface dressing occursimmediately after the chippings have been spread on thebinder film. At this stage the chippings have yet to become aninterlocking mosaic and are held in place solely by theadhesion of the binder film. Dusty chippings can seriouslyimpede adhesion and can cause immediate failure of thedressing.

3.8 The effect of dust can sometimes be mitigated bydampening them prior to spreading them on the road. Thechippings dry out quickly in contact with the binder and,when a cutback bitumen or emulsion is used, good adhesiondevelops more rapidly than when the coating of dust is dry.

3.9 Most aggregates have a preferential attraction for waterrather than for bitumen. Hence if heavy rain occurs within thefirst few hours when adhesion has not fully developed, loss ofchippings under the action of traffic is possible. where wetweather damage is considered to be a severe risk, or theimmersion tray test, described in Appendix C, shows that thechippings have poor affinity with bitumen, an adhesion agentshould be used. An adhesion agent can be added to the binderor, used in a dilute solution to pre-coat the chippings.However, the additional cost of the adhesion agent will bewasted if proper care and attention is not given to all otheraspects of the surface dressing process.

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3.10 Improved adhesion of chippings to the binder film canalso be obtained by pre-treating the chippings beforespreading. This is likely to be most beneficial if the availablechippings are very dusty or poorly shaped, or if trafficconditions are severe. There are basically two ways of pre-treating chippings:

• Spraying the chippings with a light application ofcreosote, diesel oil, or kerosene at ambient temperature(NAASRA, 1986). This can be conveniently done as thechippings are transferred from stockpile to gritting lorriesby a belt conveyor or. alternatively, they can be mixed in asimple concrete mixer.

• Pre-coating the chippings with a thin coating of hardbitumen such that the chippings do not stick together andcan flow freely.

3.11 Chippings which are pre-coated with bitumenenable the use of a harder grade of binder for constructionwhich can provide early strong adhesion and thus help toobtain high quality dressings. The binder used for pre-coatingneed not necessarily be the same kind as that used for thesurface dressing; for example, tar-coated chippings adherewell to a sprayed bitumen film. Pre-coating is usuallyundertaken in a hot-mix plant and the hardness of the coating,and thus the tendency for the chippings to adhere to eachother, can be controlled by the mixing temperature and/or theduration of mixing; typical coating temperature are about1400C for bitumen binders and 1200C for tar binders. Table 1indicates the amount of binder recommended for lightlycoating chippings.

Table 1 Binder contents for lightly-coated chippings

Target binder content(per cent by mass)

Nominal size of Bitumen Tarchippings (mm) (TRL, 1996) (TRL, 1992)

6 1.0 1.210 0.8 1.014 0.6 0.820 0.5 0.7

Reproduced from Road Note 39

3.12 Pre-coated chippings should not be used withemulsions because the breaking of the emulsion will beadversely affected.

3.13 In some countries adhesion agents or pre-treatedchippings are often used in an attempt to counteract theadverse effect of some fundamental fault in the surfacedressing operation. If loss of chippings has occurred, it isadvisable to check whether the viscosity of the binder wasappropriate for the ambient road temperature at the time ofspraying. The effectiveness of the chipping and traffic controloperations should also be reviewed

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before the use of an adhesion agent or pre-treated chippings isconsidered.

4 Bitumens

4.1 It is essential that good bonding is achieved between thesurface dressing and the existing road surface. This means thatnon-bituminous materials must be primed before surfacedressing is carried out.

Prime coats

4.2 Where a surface dressing is to be applied to a previouslyuntreated road surface it is essential that the surface should bedry, clean and as dust-free as possible. On granular, cement orlime-stabilised surfaces a prime coat of bitumen ensures thatthese conditions are met. The functions of a prime coat can besummarised as follows.

• It assists in promoting and maintaining adhesion betweenthe roadbase and a surface dressing by pre-coating theroadbase and penetrating surface voids.

• It helps to seal the surface pores in the roadbase thusreducing the absorption of the first spray of binder of thesurface dressing.

• It helps to strengthen the roadbase near its surfa ce bybinding the finer particles of aggregate together.

• If the application of the surface dressing is delayed forsome reason it provides the roadbase with a temporaryprotection against rainfall and light traffic until thesurfacing can be laid.

4.3 The depth of penetration of the prime should be between3-l0mm and the quantity sprayed should be such that thesurface is dry within a few hours. The correct viscosity andapplication rate are dependent primarily on the texture anddensity of the surface being primed. The application rate is.however, likely to lie within the range 0.3-1.1 kg/m2 . Lowviscosity cutbacks are necessary for dense cement or lime-stabilised surfaces, and higher viscosity cutbacks for untreatedcoarse-textured surfaces. It is usually beneficial to spray thesurface lightly with water before applying the prime coat asthis helps to suppress dust and allows the primer to spreadmore easily over the surface and to penetrate. Bitumenemulsions are not suitable for priming as they tend to form askin on the surface.

4.4 Low viscosity, medium curing cutback bitumens such asMC-30, MC-70, or in rare circumstances MC-250, can beused for prime coats (Asphalt Institute, 1983). Therelationship between grade and viscosity for cutback primes isshown in Table 2.

Table 2 Kinematic viscosities of current cutback binders

Permitted viscosity rangeGrade of cutback binder (centistokes at 60ºC)

MC 250 250-500MC 70 70-140MC 30 30-60ASTM D2027, 1998

Bitumens for surface dressings4.5 The correct choice o f bitumen for surface dressingwork is critical. The bitumen must fulfil a number ofimportant requirements. They must:

• be capable of being sprayed;

• 'wet' the surface of the road in a continuous film;

• not run off a cambered road or form pools of binder inlocal depressions;

• 'wet' and adhere to the chippings at road temperature;

• be strong enough to resist traffic forces and hold thechippings at the highest prevailing ambienttemperatures;

• remain flexible at the lowest ambient temperature,neither cracking nor becoming brittle enough to allowtraffic to 'pick-off' the chippings; and

• resist premature weathering and hardening.

4.6 Some of these requirements conflict. hence theoptimum choice of binder involves a careful compromise. Forexample, the binder must be sufficiently fluid at roadtemperature to 'wet' the chippings whilst being sufficientlyviscous to retain the chippings against the dislodging effect ofvehicle tyres when traffic is first allowed to run on the newdressing.

4.7 Figure 2 shows the permissible range of binderviscosity for successful surface dressing at various roadsurface temperatures. In the tropics, daytime roadtemperatures typically lie between about 250C and 500C,normally being in the upper half of this range unless heavyrain is falling. For these temperatures the viscosity of thebinder should lie between approximately l04 and 7 x l05

centistokes. At the lower road temperatures cutback grades ofbitumen are most appropriate. whilst at higher roadtemperatures penetration grade bitumens can be used.

4.8 The temperature/viscosity relationships shown inFigure 2 do not apply to bitumen emulsions. These have arelatively low viscosity and 'wet' the chippings readily, afterwhich the emulsion 'breaks',

the water evaporates. and particles of high viscosity bitumenadhere to the chippings and the road surface.

4.9 Depending upon availability and local conditions atthe time of construction, the following types of bitumen areeither commonly used in the tropics or are becoming so:

• Penetration grade.

•• Cutback.

• Emulsion.

• Modified bitumens.

Penetration grade bitumens4.10 Penetration grade bitumens vary between 80/100 to approximately 700 penetration. The softer penetrationgrade binders are usually produced at the refinery but can bemade in the field by blending appropriate amounts ofkerosene, diesel, or a blend of kerosene and diesel. Withhigher solvent contents the binder has too low a viscosity tobe classed as being of penetration grade and is then referred toas a cutback bitumen which, for surface dressing work, isusually an MC or RC 3000 grade. In very rare circumstancesa less viscous grade such as MC or RC 800 may be used if thepavement temperature is below 150C for long periods of theyear.

Bitumen emulsion4.11 Cationic bitumen emulsion with a bitumen content of70 to 75 per cent is recommended for most surface dressingwork. This type of binder can be applied through whirlingspray jets at a temperature between 70 and 850C and, onceapplied, it will break rapidly on contact with chippings ofmost mineral types. The cationic emulsifier is normally anantistripping agent and this ensures good initial bondingbetween chippings and the bitumen.

4.12 When high rates of spray are required, the road is on agradient, or has considerable camber, the emulsion is likely todrain from the road or from high parts of the road surfacebefore 'break' occurs. In these cases it may be possible toobtain a satisfactory result if the bitumen application is 'split',with a reduced initial rate of spray and a heavier applicationafter the chippings have been applied. If the intention was toconstruct a single seal then the second application of binderwill have to be covered with sand or quarry fines to preventthe binder adhering to roller and vehicle wheels. If a doubledressing is being constructed then it should be possible toapply sufficient binder in the second spray to give therequired total rate of spray for the finished dressing.

4.13 if split application of the binder is used care

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Figure 2 Surface temperature/choice of binder for surface dressings

must be taken with the following:

• The rate of application of chippings must be correct sothat there is a minimum of excess chippings.

• The second application of binder must be applied beforetraffic is allowed onto the dressing.

• For a single seal it will be necessary to apply grit or sandafter the second application of hinder.

Cutback bitumens4.14 Except for very cold conditions, MC or RC3000 grade cutback is normally the most fluid binder used forsurface dressings. This grade of cutback is basically an 80/100penetration grade bitumen blended with approximately 12 to17 per cent of cutter.

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4.15 In some areas of the world the range of bindersavailable to the engineer is restricted. In this situation it maythen be necessary to blend two grades together or to 'cut-back'a supplied grade with diesel oil or kerosene in order to obtaina binder with the required viscosity characteristics. Diesel oil,which is less volatile than kerosene and is generally moreeasily available, is preferable to kerosene for blendingpurposes. Only relatively small amounts of diesel oil orkerosene are required to modify a penetration grade bitumensuch that its viscosity is suitable for surface dressing at roadtemperatures in the tropics. For example Figure 3 shows that,for the road temperatures prevailing during trials in Kenya,between 2 and 10 per cent of diesel oil was required to modify80/100 pen bitumen to produce binders with viscosities withinthe recommended range for use (Figure 2). Figure 4 shows thetemperature/viscosity relationships for five of the blendsmade for these trials.

Figure 3 Blending characteristics of 80/100 pen bitumen with diesel fuel

Figure 4 Viscosity/temperature relationships for blends of 80/100 pen bitumen with diesel fuel

4.16 The blending process is not difficult but it must beundertaken with great care by staff who are properly trained.A convenient method is to pump the required amount of cutter(e.g. diesel oil) into the distributor whilst simultaneouslypumping in hot bitumen. Before pumping in the cutter,sufficient bitumen should be pumped into the distributor toenable the cutter to discharge below the surface of thebitumen. Because of the fire risk, all the burners

must be extinguished and naked lights and smoking prohibitedduring this operation.

Polymer modified bitumens4.17 Polymers can be used in surface dressing to modifypenetration grade, cutback bitumens and emulsions. Usuallythese modified binders are used at locations where the roadgeometry, traffic

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characteristics or the environment, dictate that the roadsurface experiences high stresses. Generally the purpose of thepolymers is to reduce binder temperature susceptibility so thatvariation in viscosity over the ambient temperature range is assmall as possible. Polymers can also improve the cohesivestrength of the binder so that it is more able to retain chippingswhen under stress from the action of traffic. They alsoimprove the early adhesive qualities of the binder allowing theroad to be reopened to traffic earlier than may be the case withconventional unmodified binders. Other advantages claimedfor modified binders are improved elasticity in bridginghairline cracks and overall improved durability.

4.18 Examples of polymers that may be used to modifybitumens are proprietary thermoplastic rubbers such asStyrene-Butadiene-Styrene (SBS), crumb rubber derived fromwaste car tyres and also glove rubber from domestic gloves.Latex rubber may also be used to modify emulsions. Bindersof this type are best applied by distributors fitted with slottedjets of a suitable size.

4.19 Rubber modified bitumen may consist, typically, of ablend of 80/100 penetration grade bitumen and three per centpowdered rubber. Blending and digestion of the rubber withthe penetration grade bitumen should be carried out byexperienced personnel prior to loading into a distributor. Thismust be done in static tanks which incorporate integral motordriven paddles. The blending temperature is approximately2000C.

4.20 Cationic emulsion can be modified in purpose madeplant by the addition of three per cent latex rubber. One of theadvantages of using emulsions is that they can be sprayed atmuch lower temperatures than penetration grade bitumens,which reduces the risk of partial degradation of the rubberwhich can occur at high spraying temperatures.

4.21 Bitumen modified with SBS exhibits thermoplastic qualities at high temperatures while having arubbery nature at lower ambient temperatures. With three percent of SBS, noticeable changes in binder viscosity andtemperature susceptibility occur and good early adhesion ofthe chippings is achieved. SBS can be obtained in a carrierbitumen in blocks of approximately 20kg mass. The blockscan be blended, at a concentration recommended by themanufacturer, with 80/100 penetration binder in a distributor.In this procedure it is best to place half of the requiredpolymer into the empty distributor, add hot bitumen from amain storage tank and then circulate the binder in thedistributor tank. The remaining blocks are added after about30 minutes and then about 2 hours is likely to be required tocomplete blending and heating of the modified binder. Everyeffort should be made to use the modified bitumen on the dayit is blended.

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Adhesion agents

4.22 Proprietary additives, known as adhesionagents. are available for adding to binders to help to minimisethe damage to surface dressings that may occur in wetweather with some types of stone. When correctly used in theright proportions. these agents can enhance adhesion betweenthe binder film and the chippings even though they may bewet. The effectiveness and the amount of an additive neededto provide satisfactory adhesion of the binder to the chippingsin the presence of free water must be determined by tests suchas the Immersion Tray Test which is described in AppendixC.

4.23 Fresh hydrated lime can also be used to enhanceadhesion. It can be mixed with the binder in the distributorbefore spraying (slotted jets are probably best suited for this)or the chippings can be pre-coated with the lime just beforeuse. by spraying with a lime slurry. The amount of lime to beblended with the bitumen should be determined in laboratorytrials bitt approximately 12 per cent by mass of the bitumenwill improve bitumen-aggregate adhesion and it should alsoimprove the resistance of the bitumen to oxidative hardening(Dickinson,1984).

4.24 Cationic emulsions inherently contain an adhesionagent and lime should not be used with this type of binder.

5 Design

5.1 The key stages in the surface dressing designprocedure are illustrated in Figure 5.

Existing site conditions

5.2 Selection of a suitable surface dressing system for aroad and the nominal size of chippings to be used is based onthe daily volume of commercial vehicles using each lane ofthe road and the hardness of the existing pavement surface.

5.3 With time, the action of traffic on a surface dressinggradually forces the chippings into the underlying surface,thus diminishing the surface texture. When the loss of surfacetexture reaches an unacceptable level a reseal will be requiredto restore skid resistance. The embedment process occursmore rapidly when the underlying road surface is softer, orwhen the volume of traffic, particularly of commercialvehicles, is high. Accordingly, larger chippings are requiredon soft surfaces or where traffic is heavy whilst smallchippings are best for hard surfaces. For example, on a verysoft surface carrying 1000 commercial vehicles per lane perday, 20mm chippings are appropriate, whilst on a very hardsurface such as concrete, 6mm chippings would be the bestchoice.

Figure 5 Outline procedure for design of surface dressings

5.4 Guidance on the selection of chipping size for singlesurface dressings, relating the nominal size of chipping to thehardness of the underlying road surface and the weight oftraffic expressed in terms of the number of commercialvehicles carried per lane per day. These recommendations areshown in Table 3.

5.5 Road surface hardness may be assessed by asimple penetration probe test (TRL. 1996). This test utilises amodified soil assessment cone penetrometer and is describedbriefly in Appendix D. Alternatively the hardness of theexisting road surface may be

made on the basis of judgement with the help of the definitionsgiven in Table 4.

5.6 Although the recommendations for the selection ofchipping size were developed for conditions in the UnitedKingdom they have been found to be applicable to roads intropical and sub-tropical countries.

5.7 If larger sized chippings are used than is recommendedin Table 3 then the necessary bitumen spray rate, required tohold the chippings in place, is likely to be underestimated by thedesign procedure

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Table 3 Recommended nominal size of chippings (mm)

Approximate number of commercial vehicles with an unladen weightgreater than 1.5 tonnes currently carried per day in the design lane

Type of surface 2000-4000 1000-2000 200-1000 20-200 Less than 20

Veryhard 10 10 6 6 6Hard 14 14 10 6 6Normal 20Ø 14 10 10 6Soft * 20Ø 14 14 10Very soft * * 20Ø 14 10

The size of chipping specified is related to the mid – point of each lane traffic category. Lighter traffic conditions may, make thenext smaller size of stone more appropriate.

Ø Very particular care should be taken when using 20mm chippings to ensure that no loose chippings remain on thesurface when the road is opened to unrestricted traffic as there is a high risk of windscreen breakage.* Unsuitable for surface dressing.

Table 4 Categories of road surface hardness

CategoryPenetration1

of surface at 300C (mm) Definition

Very hard 0-2 Concrete or very lean bituminous structures with dry stony surfaces. There would benegligible penetration of chippings under the heaviest traffic.

Hard 2-5 Likely to be an asphalt surfacing which has aged for several years and is showing somecracking. Chippings will penetrate only slightly under heavy traffic.

Normal 5-8 Typically, an existing surface dressing which has aged but retains a dark and slightlybitumen-rich appearance. Chippings will penetrate moderately under medium and heavytraffic.

Soft 8-12 New asphalt surfacings or surface dressings which look bitumen-rich and have onlyslight surface texture. Surfaces into which chippings will penetrate considerably undermedium and heavy traffic.

Very soft >12 Surfaces, usually a surface dressing which is very rich in binder and has virtually nosurface texture. Even large chippings will be submerged under heavy traffic.

'See Appendix D

described in Section 5. This is likely to result in the 'whip-off' ofchippings by traffic early in the life of the dressing and also tohave a significant effect on the long term durability of lowvolume roads.

5.8 In selecting the nominal size of chippings for doublesurface dressings, the size of chipping for the first layer should beselected on the basis of the hardness of the existing surface andthe traffic category as indicated in Table 3. The nominal size ofchipping selected for the second layer should preferably have anALD of not more than half that of the chippings used in the firstlayer. This will promote good interlock between the layers.

5.9 In the case of a hard existing surface, where verylittle embedment of the first layer of chippings is possible, such asa newly constructed cement stabilised road base or a densecrushed rock base, a 'pad coat' of 6mm chippings should beapplied first followed by l0mm or 14 mm chippings in the second

12

layer. The first layer of small chippings will adhere well to thehard surface and will provide a key' for the larger stone of thesecond dressing.

Selecting the binder5.10 The selection of the appropriate binder for asurface dressing is usually constrained by the range of bindersavailable from suppliers, although it is possible for the user tomodify the viscosity of penetration grade and cutback binders tosuit local conditions as described in paragraphs 4.14 to 4.16.

5.11 The factors to be taken into account in selecting anappropriate binder are:

•• The road surface temperature at the time the surfacedressing is undertaken. For penetration grade and cutbackbinders the viscosity of the binder should be between 1 0~and 7x 10~ centistokes at the road surface temperature(see paragraphs 4.5 to 4.9).

• The nature of the chippings. If dusty chippings areanticipated and no pre-treatment is planned, the viscosityof the binder used should be towards the lower end of thepermissible range. if the binder selected is an emulsion itshould be borne in mind that anionic emulsions may notadhere well to certain acidic aggregates such as graniteand quartzite.

• The characteristics of the road site. Fluid binders such asemulsions are not suited to steep cross falls or gradientssince they may drain off the road before 'breaking'.However, it may be possible to use a 'split application' ofbinder.

• The type of binder handling and spraying equipmentavailable. The equipment must be capable of maintainingan adequate quantity of the selected binder at itsappropriate spraying temperature and spraying it evenlyat the required rate of spread.

• The available binders. There may be limited choice ofbinders but a balanced choice should be made wherepossible. Factors which may influence the final selectionof a binder include cost, ease of use, flexibility withregard to adjusting binder viscosity on site and anyinfluence on the quality of the finished dressing.

5.12 Consideration of these factors will usually narrow thechoice of binder to one or two options. The final selection willbe determined by other factors such as the past experience ofthe surface dressing team.

Choice of binder and timing of construction work

5.13 The choice of cutback grade or penetrationgrade bitumen for surface dressing work is largely controlledby road temperatures at and shortly after the time ofconstruction. However, there are relative advantages anddisadvantages associated with the use of penetration gradebinders or cutback bitumen.

5.14 MC 3000 cutback binder typically contains 12 to 17per cent of cutter. Under warm road conditions this makes thebinder very tolerant of short delays in the application ofchippings and of the use of moderately dusty chippings. It istherefore a good material to use for training new surfacedressing teams and for use in areas where water for cleaningchippings is scarce. However, a substantial percentage of thecutter, especially if it is diesel, can remain in the seal for manymonths. If road temperatures increase soon after construction,it is likely that MC3000 will be found to be 'tender' and thatthe seal can be easily damaged. This should not be a problemfor lightly trafficked roads and for new roads that are notopened to general traffic for several days after the surfacedressing is constructed. If a road must be opened to fast highvolume traffic within a few hours of construction then therewill be considerable advantage in using as high a viscosity

binder as conditions will permit. For instance, if the roadtemperature is 400C then for heavy traffic the chart in Figure2 would suggest that MC 3000 would be only just viscousenough. 400/500 penetration grade bitumen would be on thelimit of being too viscous, however, it would be preferable tocut-back the bitumen to a 500/600 penetration grade ratherthan use a MC3000 grade. If pre-coated chippings could beused then the use of a 400 penetration grade bitumen wouldbe acceptable.

5.15 Penetration grade bitumens as hard as 80/100 are o ftenused for surface dressing work when road temperatures arehigh. With such a high viscosity bitumen it is very importantthat the chippings are applied immediately after spraying and,to achieve this, the chipping spreader must follow closelybehind the distributor. The construction team must be wellorganised and skilful. This type of binder will not be tolerantof delays in the application of the chippings nor of the use ofdusty chippings. In either situation, early trafficking is verylikely to dislodge chippings and seriously damage the seal.

5.16 The use of penetration grade binders in the range80/100 to 400 is preferred to MC3000 wherevercircumstances allow this. For high volume fast traffic, wherevery early adhesion of the chippings is essential,consideration should be given to the use of pre-coatedchippings. This will allow the use of a more viscous binderfor a given road temperature and will ensure that a strongearly bonding of the chippings is obtained. A polymermodified or rubberised binder can also provide immediatestrong adhesion. Alternatively, emulsions will provide good'wetting' and early adhesion provided rainfall does notinterfere with curing.

5.17 The most difficult situations occur when it is requiredto start work early in the day and temperatures areconsiderably lower than they will be in the afternoon. It mayappear to be appropriate to use a cutback binder, such asMC3000, for the low road temperature but, by the afternoon,the seal is likely to be too 'soft'. In these situations it better touse a more viscous binder and keep the traffic off of the newseal until it has been rolled in the afternoon.

Designing the surface dressing

Basis for the design method

5.18 Having selected the nominal size of chipping and thetype of binder to be used, the next step in the design of asurface dressing is to determine the rate of spread of thebinder. In this respect the recommendations given in RoadNote 39 (TRL, 1996) for conditions in the United Kingdomare not appropriate for most tropical or sub-tropical countries.Differences in climate, uniformity of road surfaces, thequality of aggregates, traffic characteristics and constructionpractice, necessitate a more general

13

approach to the determination of the rate of spread of thebinder for application in tropical countries.

5.19 The method of surface dressing design put forwardby Jackson (1963) is suitable for general application andtrials undertaken by the TRL in Kenya (Hitch, 1981)indicate that with some minor modifications, it works wellunder a range of tropical and sub-tropical conditions.Accordingly this method is recommended as a good basison which to develop national or regional standards forsurface dressing design in tropical countries.

5.20 The Jackson method of design incorporatesconcepts first put forward by Hanson (1934) which relatethe voids in a layer of chippings to the amount of bindernecessary to hold the chippings in place. Hanson calculatedthat in a loose single layer of chippings, such as is spreadfor a surface dressing, the voids are initially about 50 percent decreasing to about 30 per cent after rolling andsubsequently to 20 per cent by the action of traffic. For bestresults, between 50 and 70 per cent of the voids in thecompacted aggregate should be filled with binder.

Hence it is possible to calculate the amount of binder requiredto retain a layer of regular, cubical chippings of any size.However, in practice chippings are rarely the ideal cubicalshape (especially when unsuitable crushing plant has beenused) and this is why the ALD concept was originallyintroduced.

Determining the average least dimension of chippings5.21 The ALD of chippings is a function of both theaverage size of the chippings, as determined by normal squaremesh sieves, and the degree of flakiness. The ALD may bedetermined in two ways.

Method A. A grading analysis is performed on arepresentative sample of the chippings in accordance withBritish Standard 812:1985. The sieve size through which 50per cent of the chippings pass is determined (i.e. the mediansize'). The flakiness index is then also determined inaccordance with British Standard 812:1985. The ALD of thechippings is then derived from the nomograph shown inFigure 6.

Figure 6 Determination of average least dimension14

Method B. A representative sample of the chippings iscarefully subdivided (in accordance with BritishStandard 812:1985) to give approximately 200chippings. The least dimension of each chipping ismeasured manually and the mean value, or ALD, iscalculated.

Determining the overall weighting factor5.22 The ALD of the chippings is used with an overallweighting factor to determine the basic rate of spray ofbitumen. The overall weighting factor F' is determined byadding together four factors that represent: the level oftraffic, the condition of the existing road surface, theclimate and the type of chippings that will be used. Factorsappropriate to the site to be surface dressed are selectedfrom Table 5.

Table 5 Weighting factors for surface dressing design

Description Factor

Total traffic (all classes)Vehicles/lane/day

Very light 0 - 50 +3Light 50 - 250 +1Medium 250 - 500 0Medium-heavy 500 - 1500 -lHeavy 1500 - 3000 -3Very heavy 3000+ -5

Existing surfaceUntreated or primed base +6Very lean bituminous +4Lean bituminous 0Average bituminous -1Very rich bituminous -3

Climatic conditionsWet and cold +2Tropical (wet and hot) +1Temperate 0Semi-arid (hot and dry) -lArid (very dry and very hot) -2

Type of chippingsRound/dusty +2Cubical 0Flaky (see Appendix A) -2Pre-coated -2

5.23 For example. if flaky chippings (factor -2) are to beused at a road site carrying medium to heavy traffic (factor- 1) and which has a very rich bituminous surface (factor -3) in a wet tropical climate (factor +1) the overallweighting factor 'F' is:

-2-1-3 + 1 = -5

5.24 The rating for the existing surface allows for theamount of binder which is required to fill the surface voidsand which is therefore not available to contribute to thebinder film that retains the chippings. If the existing surfaceof the road is rough, it should be rated as 'very leanbituminous' even if its overall colour is dark with bitumen.Similarly, when determining the rate of spread of binder forthe second layer of a double surface dressing, the first layershould also be rated 'very lean bituminous'.

5.25 The Jackson method of determining the rate ofspread of binder requires the estimation of traffic in terms ofnumbers of vehicles only. However, if the proportion ofcommercial vehicles in the traffic stream is high (say morethan 20 per cent) the traffic factor selected should be for thenext higher category of traffic than is indicated by the simplevolume count.

Determining the basic bitumen spray rate5.26 Using the ALD and 'F' values in equation 1 willgive the required basic rate of spread of binder.

R = 0.625+(F*0.023)+[0.0375+(F*0.0011)]ALD (1)

Where F = Overall weighting factor

ALD = the average least dimension of thechippings (mm)

R = Basic rate of spread of bitumen(kg/m2)

5.27 Alternatively, the two values can be used in thedesign chart given in Figure 7. The intercept between theappropriate factor line and the ALD line is located and therate of spread of the binder is then read off directly at thebottom of the chart. The basic rate of spread of bitumen (R)is the mass of MC3000 binder per unit area on the roadsurface immediately after spraying. The relative density ofMC3000 can be assumed to be 1.0 and the spread rate cantherefore also be expressed in 1/m2, however, calibration of adistributor is easier to do by measuring spray rates in termsof mass.

Spray rate adjustment factors5.28 Research in Kenya (Hitch, 1981) and elsewhere, hasindicated that best results will be obtained if the basic rate ofspread of binder is adjusted to take account of traffic speedand road gradient as follows.

• For slow traffic or climbing grades with gradientssteeper than 3 per cent, the basic rate of spread ofbinder should be reduced by approximately 10 percent.

• For fast traffic or downgrades steeper than 3 per centthe basic rate of spread of binder should be increasedby approximately 10 per cent.

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Figure 7 Surface dressing design chart

5.29 The definition of traffic speed is not precise but ismeant to differentiate between roads with a high proportionof heavy vehicles and those carrying mainly cars travellingat 80km/h or more.

5.30 The basic rate of spread of binder must also bemodified to allow for the type of binder used. Thefollowing modifications are appropriate:

• Penetration grade binders: decrease the rate of spreadby 10 per cent.

• Cutback binders: for MC/RC 3000 no modification isrequired. (In the rare cases when cutbacks with lowerviscosity are used the rate of spread should beincreased to allow for the additional percentage ofcutter used).

• Emulsion binders: multiply the rate of spread given inthe chart by 90/bitumen content of the emulsion (percent). This calculation includes a reduction of ten percent for the residual penetration grade binder.

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5.31 Suggested adjustment factors for different bindersand different site conditions are given in Table 6. Theadjustment factors reflect the amount of cutter used in thebase 80/100 penetration grade bitumen but must be regardedas approximate values.

5.32 The amount of cutter required for 'on-site' blendingshould be determined in the laboratory by making viscositytests on a range of blends of bitumen and cutter. Work atTRL (Hitch and Stewart, 1987) has shown that MC3000 canbe made in the field by blending 90 penetration bitumen with12 to 14 per cent by volume of a 3:1 mixture of kerosene anddiesel. It is suggested that if there is significantly more than14 per cent of cutter by volume then the spray rate should beadjusted to compensate for this. For binders which have beencutback at the refinery, the cutter content should be obtainedfrom the manufacturer.

5.33 If a different grade of binder is required thenthe adjustment factor should reflect the differentamount of cutter used. For instance, a 200 penetration

Table 6 Typical bitumen spray rate adjustment factors

Basic spray rate from Flat terrain, moderate High speed traffic, Low speed traffic,Binder grade Figure 7 or equation 1 traffic speeds down-hill grades >3% up-hill grades >3%

MC3000 R R R*l.1 R*0.9300 pen R R*0.95 R~1.05 R*0.8680/100 pen R R*0.9 R*0.99 R*0.81Emulsion1 R R*(90/%binder) R*(99/%binder) R*(8l/%binder)

1 ‘% binder’ is the percentage of bitumen in the emulsion.

binder may have 3 per cent cutter in it and therefore thespray rate is 103 per cent of the rate for a 80/100penetration bitumen. Appendix E gives an example of theuse of the design chart and adjustment factors.

Adjusting rates of spray for maximum durability5.34 The spray rate which will be arrived at afterapplying the adjustment factors in Table 6 will provide verygood surface texture and use an 'economic' quantity ofbinder. However, because of the difficulties experienced inmany countries in carrying out effective maintenance, thereis considerable merit in sacrificing some surface texture forincreased durability of the seal. For roads on flat terrain andcarrying moderate to high speed traffic it is possible toincrease the spray rates obtained by applying the factorsgiven in Table 6 by approximately 8 per cent. The heavierspray rate may result in the surface having a 'bitumen-rich'appearance in the wheel paths of roads carrying appreciablevolumes of traffic. However, the additional binder shouldnot result in bleeding and it can still be expected that moresurface texture will be retained than is usual in an asphaltconcrete wearing course.

Surface dressing design for low volume roads5.35 if a low volume road, carrying less than about100 vehicles per day, is surface dressed it is very importantthat the seal is designed to be as durable as possible tominimise the need for subsequent maintenance.

5.36 A double surface dressing should be used on newroadbases and the maximum durability of the seal can beobtained by using the heaviest application of bitumenwhich does not result in bleeding.

5.37 Where crushing facilities are put in place solely toproduce chippings for a project, it will be important tomaximise use of the crusher output. This will require theuse of different combinations of chipping sizes andcorrespondingly different bitumen spray rates. Thenormally recommended sizes of chippings for differentroad hardness and low commercial traffic volumes arereproduced in Table 7.

5.38 It may be desirable to use chippings of a larger sizethan those recommended in Table 7 for reasons ofeconomy. It is likely that the rate of application of

Table 7 Nominal size of chippings for different hardness

of road surface

No. of commercialVehicles/lane/day1 20-100 <20

Category of roadsurface hardness Nominal chipping size mm)

Very hard 6 6Hard 6 6Normal 10 6Soft 14 10

1 Vehicles with an unladen weight greater than 1.5 tonnes

bitumen determined in the normal way will be too low toobtain good durability. Low volumes of traffic are alsounlikely to cause the chippings to be 'rotated' into a tightmatrix and this will result in the layer being of greater depththan the ALD of the chippings, which is assumed in thedesign process. It should therefore be safe to increase bitumenspray rates on low volume roads to compensate for thereduced embedment of 'oversize' chippings and the increasedtexture depth that results from less re orientation of thechippings under light traffic

5.39 Ideally the ALD of the two aggregate sizes used in adouble surface dressing should differ by at least a factor oftwo. If the ALD of the chippings in the second seal is morethan half the ALD of the chippings in the first seal then thetexture depth will be further increased and the capacity of theaggregate structure for bitumen will be increased.

5.40 It is suggested that on low volume roads the bitumenspray rates should be increased above the basic rate of spreadof bitumen (see paragraphs 5.26 to 5.27) by up to thepercentages given in Table 8. It is important that theseincreased spray rates are adjusted on the basis of trial sectionsand local experience.

Spread rate of chippings5.41 An estimate of the rate of application of the chippingsassuming that the chippings have a loose density of1.35Mg/m3, can be obtained from the following equation:

Chipping application rate (kg/m2) - 1 .364*ALD

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Table 8 Suggested maximum increases in bitumen spray rate for low volume roads

ALD of chippings (mm) 3 6 >6

All traffic (vehicles/lane/day) <20 20-100 <20 20-100 <20 20-100

Increase in bitumen spray rate (per cent) 15 10 20 15 30 20

5.42 The chipping application rate should be regarded asa rough guide only. It is useful in estimating the quantity ofchippings that is required for a surface dressing projectbefore crushing and stockpiling of the chippings is carriedout. A better method of estimating the approximateapplication rate of the chippings is to spread a single layerof chippings taken from the stockpile on a tray of knownarea. The chippings are then weighed, the process repeatedten times with fresh chippings, and the mean valuecalculated. An additional ten per cent is allowed for whipoff. Storage and handling losses must also be allowed forwhen stockpiling chippings.

5.43 The precise chipping application rate must bedetermined by observing on site whether any exposedbinder remains after spreading the chippings, indicating toolow a rate of application of chippings, or whether chippingsare resting on top of each other, indicating too high anapplication rate. Best results are obtained when thechippings are tightly packed together, one layer thick. Toachieve this. a slight excess of chippings must be applied.Some will be moved by the traffic and will tend to fill smallareas where there are insufficient chippings. Too great anexcess of chippings will increase the risk of whip-off andwindscreen damage.

6 Plant and equipment

Methods of distributing binder6.1 The success of a surface dressing is very dependenton the binder being applied uniformly at the correct rate ofspread. The method adopted for distributing binder musttherefore;

• be capable of spreading the binder uniformly and atthe predetermined rate of spread; and

• be able to spray a large enough area in a working dayto match the required surface dressing programme.

6.2 The use of hand-held containers such as wateringcans, perforated buckets etc, has a place for minor works.Any type of binder from penetration grades to emulsion canbe applied in this way but uniform spreading ofpredetermined amounts cannot be achieved by this methodand hence it is not recommended for anything other thansmall-scale work. A rather more controllable method ofhand

18

application is to use hand lances. if skilfully used, they canproduce an acceptably uniform rate of spread but it is verydifficult to achieve a specified rate of spread with them. Theycannot therefore be recommended for other than small-scalework and limited maintenance operations. The use of either ofthese hand methods of binder application for larger scale workinvariably results in waste of valuable binder and a poorquality surface dressing which will have a short 'life'.

6.3 The spreading of binder on a larger scale requires theuse of a bulk binder distributor, which may be either a selfpropelled or a towed unit (British Standards BS 1707:1989,and BS 3136:Part 2:1972)

6.4 There are two basic types of bulk binder distributors,the pressurised tank, constant rate of spread, constant volume,and constant pressure machines.

Constant volume distributors6.5 These distributors are fitted with positive displacementpumps, the output of which can be pre-set. All the binderdelivered by the pump is fed to the spray-bar when spraying isin progress and there is no by-pass arrangement for re-circulating binder to the tank. For a spray bar of given lengthand output, the rate of spread of binder on the road isinversely proportional to the forward road speed of thedistributor. On most constant volume machines it is possibleto preheat the spray bar by circulating hot binder to it beforespraying commences but this facility is not available on allmachines.

6.6 Constant volume distributors can spray a wide range oftypes of binder and they are quite common in tropicaldeveloping countries. Disadvantages of constant volumedistributors are;

• Calibration involves three inter-related variables, i.e. thepump output, the road speed and the spray bar width;hence the calibration procedures need to be extensive if,for example, it is required to vary spray bar width toallow for different lane widths. However, some constantvolume machines have a limited but useful degree ofautomatic control of bitumen pump speed to compensatefor variation in road speed.

• The relative mechanical complexity of the machinesmeans that they are not suitable for operation by partlyskilled operators.

6.7 Most distributors manufactured in the USA areconstant volume machines.

Constant pressure distributors6.8 In these machines a pump of adequate capacitydelivers binder to the spray bar at a pre-set pressure. Arelief valve regulates the pressure and permits binder tobypass the spray bar and return to the tank. The pressure inthe spray bar is not affected by the number of jets in use,and hence re-calibration is not required when spray barextensions are fitted or the number of jets are reduced. Aswith constant volume machines, the rate of spread of bindervaries inversely with the road speed of the distributor.

6.9 Most distributors made in the UK are of the constantpressure type.

Principal components of binder distributors

6.10 Distributors spray the bitumen through a spray barto which the binder is delivered by a pump. or underpressure, from a heated insulated storage tank. Briefdescriptions of these principal components and generalguidelines on their operation are given below.Manufacturers’ instruction manuals give detailed operatinginstructions for each model of distributor. These should becarefully followed and used to train operators so that theyfully understand the principles and the correct method ofoperation of their distributor.

Spray bars and spray jets6.11 There are basically two types of spray jets, slottedjets and whirling spray jets. Slotted jets are usually highoutput jets and are particularly suitable for sprayingpolymer modified binders or for grouting. However, somemanufacturers can supply jets with a range of different slotwidths or whirling spray jets for the same spray bar.Whirling spray jets are of lower output and have theadvantage for normal surface dressing in that the forwardspeed of the distributor can be slower than when slotted jetsare used. This can enable the speed to be controlled moreeasily and for the chipping operation to keep pace with thespraying. Higher bitumen temperatures are necessary whenspraying with whirling jets and suitable sprayingtemperatures for both types of jets are given in Table 9.

6.12 The swirl chamber of whirling spray jets is enclosedin the spray bar so that the jets can be pre-heated effectivelyby circulating hot binder through the spray bar prior tospraying. The fine spray produced by whirling spray jetsnecessitates protecting the spray bar with a hood andcanvas curtains to prevent wind from deflecting the spray.This is not required with slotted jets.

Table 9 Spraying temperatures for binders

Whirling spray jets Slotted jets

Cutback grades Min ºC Max ºC Min ºC Max ºC

MC30 50 60 40 50RC/MC70 65 80 55 70RC/MC250 95 115 80 90RC/MC800 115 135 105 115RC/MC3000 135 150 120 130

Penetration grades400/500 160 170 140 150280/320 165 175 150 160180/200 170 190 155 16580/100 180 200 165 175

1 Because of the flammable nature of the solvent used inRC-type cutbacks, application temperatures should berestricted to the lower parts of the ranges given above.

2 It is essential to extinguish flames and prohibit smokingwhen heating, pumping or spraying all cutbacks. Fireextinguishers should always be readily at hand.

6.13 On constant pressure distributors a pressure gaugefitted to the spray bar registers the spraying pressure duringspraying (though not when re-circulating only), and on somemachines a temperature gauge is also fitted to the spray bar.

6.14 The uniformity of transverse distribution of a spraybar should be checked by the 'Depot tray test' at least once ayear. This test is specified fully in BS 1707:1989 (1989) andis described in Appendix F.

6.15 Attention should also be paid to maintaining thecorrect height of the spray bar above the road. Whilst jets arepositioned on the spray bar so that their sprays overlap tominimise the effect of variations in spray bar height on theuniformity of transverse distribution of binder. some adverseeffects are likely if the spray bar is operated at an incorrectheight. Slotted jets are more critical than whirling spray jets inthis respect.

6.16 Since the spray of the last jet at each end of a spray baris not overlapped by an adjacent spray the rate of spread ofbinder is less at the ends of a spray bar than along its length.For this reason, adjacent spraying runs of a distributor arenormally overlapped. Some distributors are fitted with a largerjet at the end of the spray bar to compensate for this effect.The alternative practice of turning the last jet of a slotted jetspray bar at right angles is not recommended, nor is thepractice of attempting to spray butt joints. This invariablyresults in narrow unsprayed strips between adjacent paths ofthe distributor.

6.17 To ensure satisfactory performance of the spray bar,strainers and in-line filters in the binder feed system must becleaned regularly otherwise blocked jets will result. Beforecommencing

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spraying, the spray bar and jets should be preheated bycirculating hot binder and then the jets should be operatedfor a few seconds, discharging on to waste ground, toensure they are operating freely.

6.18 If spraying is interrupted briefly. for example, toallow the chipping operation to catch up, the spray barshould be kept hot by circulating binder, preferably withthe distributor standing off the road. When spraying isstopped for a longer period, such as at the end of the day orwhen the tank is being re-filled, the binder pump should beopened to air and the feed line to the spray bar, the spraybar itself, and the return line back to the tank emptied. Thereturn valve should then be closed and the jets blown outwith air. If the machine is being allowed to cool completelythe binder pump should be flushed out with diesel fuel.Most spray bars are fitted with a drain cock so that binderor flushing oil can be drained off when required.

Binder pumps and air pumps6.19 On most distributors the binder pump is driven by aseparate engine, usually mounted either at the rear of thetank or between the tank and the driving cab. The pumpitself is normally located inside the binder tank so that it iskept hot by the surrounding binder. The engine drive to thepump is usually through a clutch and the same engineusually drives a small air compressor which supplies airand fuel under pressure to the burners.

6.20 The binder tank should be emptied at the end of aday's work so that when the tank is next filled with hotbinder there is no cold binder around the pump to prevent itfrom warming up quickly. If the binder system is notcleaned out as described above the pump will not workuntil it has been cleared of cold bitumen. This should bedone by turning the engine crank manually as the bitumenin the distributor is heated and not by using the engine.

6.21 On some distributors the pump drives are takeneither from the main power transmission of the vehicle orare driven by the main engine through a hydraulic system.

Tanks and burners6.22 Most binder distributors have tanks with a capacityof between 500 and 16000 litres. The tanks are invariablymade of steel and are lagged to reduce heat loss. Baffles arefitted internally to minimise surge. An inspection hatchfitted with a strainer basket provides access at the top of thetank, and a dipstick or contents gauge indicates the level ofthe binder.

6.23 Flues fitted with burners run through the tank toheat the binder and a thermometer is fitted to indicate thetemperature. The burners use either

20

kerosene or diesel fuel which is usually drawn from the mainfuel tank of the vehicle. Vaporising burners require thevaporising coil to be heated before they can operate, whilstatomising burners, which are preferable. can start up fromcold. Fire extinguishers, suitable for fighting fires fuelled bybitumen or solvents, should be located in convenientpositions. It is important that professional advice is obtainedon fire-fighting matters well before work commences.

6.24 The burners in a distributor should be used to makeonly relatively small adjustments to the binder temperature.Wherever possible the main operation of heating the bindershould be done in pre-heaters and the binder transferred to thedistributor at or above the spraying temperature.

6.25 When heating binder in the tank it is necessary toensure that the burner flues are fully covered by the binder,preferably with a depth of at least 150mm of binder over thetop of the flues. On some distributors a danger level isindicated on the contents gauge. If this precaution is notobserved the burner flues may burn out, causing a fire orexplosion.

6.26 Burners must not be operated when the distributor isspraying or moving or if any blending is in progress. Toprevent 'coking' of binder in the vicinity of the flues it isrecommended that the binder is circulated when the burnersare lit. This will also speed up the transfer of heat throughoutthe binder.

Distributor speed control and calibration6.27 Most binder distributors are equipped with a 'fifthwheel' which operates a low range speedometer. Thespeedometer is located in the driver's cab in a prominentposition so that a steady forward speed can be maintainedrelatively easily.

6.28 To spray binder at a specified rate of spread all that isnecessary with constant pressure machines is to read off thecorresponding road speed from the 'Driver's chart' orcalibration chart which should be carried by every distributor.With constant volume machines it is necessary to select fromthe chart both the pump output and the road speed necessaryto give the required rate of spread for the width of spray barbeing used.

6.29 If the distributor has not previously been calibrated orif the calibration chart has been lost either of the followingtwo methods can be used to calibrate the machine.

Method A. This method is preferred for initialcalibration. The distributor is loaded with binder which israised to the correct spraying temperature and circulatedaround the spray bar to heat it. Static spraying is doneinto suitable containers to check the evenness of theappearance of the binder spray.

Binder is then sprayed into weighed containers ofsuitable dimensions for an accurately measured periodof time and the mass of sprayed binder determined byweighing. The mass of binder delivered per unit timeis calculated and the rate of spread/speed of distributorrelationship is determined as described in Appendix G.

Method B. Four or five weighed metal trays of knownarea (0.lm square is a suitable size) are placed in thepath of the distributor as it makes a spraying run at aconstant speed. The trays are then picked up andweighed and the rate of spread of binder is calculated.The process is repeated with different distributorspeeds until the required rate of spread/speed chart canbe drawn up. This tray test should be repeatedperiodically during surface dressing operations tocheck the consistency of the rate of spread of bitumen.It will, of course, be necessary to complete thedressing by hand on the areas where the trays werelocated.

6.30 Tar and bitumen binders have been found to havedifferent outflow characteristics when sprayed fromwhirling spray jets; hence, if both kinds of binder are likelyto be used, it is advisable to draw up a calibration chart forboth binders for machines fitted with this type of jet.

Chip spreaders6.31 Chippings can be spread on the sprayed binder byhand and good results can be obtained by this method witha well-trained and plentiful labour force. In general,however, better results will be obtained when chippings arespread mechanically since this facilitates a more evendistribution and rapid application of the chippings after thebinder has been sprayed.

6.32 There are three main types of chip spreader;

• Metering or non-metering 'tail-board' types.

• Pushed metering chip spreaders.

• Self-propelled metering or non metering chipspreaders.

6.33 Non-metering tail board chip spreaders arebolted in place of the tailgate of a normal tipping lorry.They are the cheapest and simplest kind of mechanical chipspreader, having very few moving parts. A serrated steelcomb controls the flow of chippings and a rotary gate witha helical edge controls the width of spread and the startingand stopping of the flow. The 'Hornsey gritter' is a popularexample of this type. The flow of chippings is controlled byan operator who walks beside the tipper lorry, whilst it isdriven in reverse at walking speed with the tipper bodypartly raised. Since the

rate of spread of the chippings is dependent on gravity and thespeed of the tipper lorry acting independently, the skill of thelorry driver is crucial in ensuring an even distribution of thechippings. Nevertheless good results can be obtained withthese simple machines.

6.34 However, to reduce dependence on the skill of thetipper driver, metering devices are available for tailboard chipspreaders that control the rate of discharge of the chippings bydelivering them over a roller which is driven from the roadwheels of the lorry or from a fifth wheel attached to the chipspreader. In this way variations in road speed of the tipperproduce corresponding variations in the rate of discharge ofthe chippings.

6.35 Pushed metering chip spreaders operate on a similarprinciple but the metering roll is located at the base of awheeled hopper which is pushed along the road by a reversingtipper lorry. The roll is driven by the road wheels of thehopper and the chippings in the hopper are replenished fromthe raised body of the tipper.

6.36 Self-propelled metering chip spreaders are the mosteffective machines available for applying chippings. Theyhave a hopper at the rear into which chippings are dischargedfrom the delivering tipper lorry which, during the transfer ofthe chippings, is towed along in reverse by the chip spreaderthrough a quick release mechanism. Conveyor belts transferthe chippings to a transverse hopper at the front of themachine at the bottom of which is the metering roll thatdelivers the chippings to the road. However, there are self-propelled models which do not meter the chippings but relyon gravity feed and these machines require careful operationto ensure that a constant road speed is maintained.

6.37 It should be noted that none of these chip spreaderscan deliver chippings at a pre-determined rate of spread; theysimply facilitate an even distribution of the chippings and theoperator must ensure that an adequate, but not excessive, rateof application is maintained.

6.38 The number of tipper lorries must be sufficient toprovide a steady supply of chippings at a rate that allows theplanned daily output of the surface dressing unit to beachieved. Depending on the distance of the stockpile ofchippings from the surface dressing site, a minimum of fouror five tippers is usually required plus one spare tipper forapplying chippings by hand to awkward shaped corners andother areas that may not have been covered by the chipspreader.

Rollers and other equipment6.39 The rolling of a surface dressing plays an importantpart in ensuring the retention of the chippings by assisting inthe initial orientation and

21

bedding down of the chippings in the binder. Traditionally,steel-wheeled rollers have been used but these tend to crushweaker aggregates and to crack poorly shaped chippings.Accordingly, if steel-wheeled rollers are used they shouldnot exceed 8 tonnes in weight and should only be used onchippings which are strong enough. Some steel-wheeledrollers are fitted with rubber sleeves which makes themmore suitable for surface dressing work but, as for anyroller of this type. they will bridge' depressions in theexisting road surface. In general. pneumatic tyred rollersare preferred because the tyres have a kneading actionwhich tends to manoeuvre the chippings into a tight mosaicwithout splitting them and they do not ‘bridge' depressions.

6.40 In favourable conditions, adhesion should be wellestablished within 30 minutes of rolling after whichconsiderable benefit can be obtained by allowing slow-moving traffic, particularly heavy lorries, to traverse thedressing provided that traffic speed is kept below 20 to 30km/hr. This is very important and the use of a lead vehicleto 'convoy' traffic at slow speed is recommended.

6.41 Other important items of equipment required forsurface dressing are mechanical brooms, binder heaters,decanters and transporters, and front-end loaders.Mechanical brooms, either towed or powered, areinvaluable for obtaining a clean road surface prior tospraying the binder. Whilst hand brooming is an alternative,it is difficult to obtain as good results by this method,particularly when sweeping the surface of a newlyconstructed roadbase from which all loose particles shouldbe removed.

6.42 Binder heaters are required to raise bulk stocks ofbinder to the spraying temperature. They should havesufficient capacity to supply. at the correct temperature, allthe binder required for the planned output of the surfacedressing unit. Binder decanters fulfil the same functionwhen the binder is supplied in drums. Their capacity tendsto be small hence it is usually necessary to provide severaldecanters to supply the required quantity of hot binder.When binder is supplied in bulk it is desirable to transport itfrom the bulk supply point in binder transporters. It is notdesirable to use distributors for this, nor should the burnersin a distributor be used for raising the binder from pumpingtemperature, at which it is usually discharged at the supplypoint, to the spraying temperature.

6.43 Front-end loaders are required primarily forhandling chippings. They are essential for loading tippinglorries quickly with chippings at the stockpile to ensure thata continuous supply of chippings is delivered to the surfacedressing site. Front-end loaders are, of course, also usefulfor many general lifting duties such as lifting drums ofbinder onto decanters. In addition to these major items of

22

equipment a surface dressing unit requires a variety of smallequipment such as road signs, hand tools etc. These are listedin Appendix H.

6.44 The routine maintenance and servicing of themechanical equipment of a surface dressing unit usually hasto be undertaken in the field, often remote from a basemaintenance workshop. Machine operators do not normallyhave the skills to undertake running repairs or adjustments totheir machines, nor are they usually authorised to do suchwork. There is thus a need to provide for a certain level ofrunning repairs on site if long delays are to be avoided whilstplant fitters and spares are supplied from a distantmaintenance workshop. The employment in the unit of a plantoperator who is also a trained filler is therefore an advantage.together with the provision on site of a basic set of spares andthe necessary tools.

7 The surface dressing process

Planning7.1 A typical sequence of events in the planning ofa surface dressing operation is as follows:

1 Select lengths of road requiring surface dressing and detailthe preliminary work required on each road before thesurface dressing can be carried out.

2 Implement the necessary preliminary work such aspatching. heating and planing, shoulder and edge repairs,drainage works. reinstatement of service trenches etc.Allow as much time as possible for trafficking beforecommencement of surface dressing operations.

3 Decide on the type of surface dressing, the binder to beused and nominal chipping sizes, i.e. use the road surfacehardness probe and make a preliminary design, taking intoaccount constraints on the supplies of binder andchippings and limitations of plant and labour.

4 Ensure that the chippings will be of adequate quality withsuitable ALD value(s). Stockpile chippings at convenientpoints along the road to be surface dressed so as tominimise haul distances during construction. Sample thechippings and confirm their suitability, modify the surfacedressing design if necessary.

5 Order the appropriate binder for the anticipated weatherconditions, or make provision for blending and addingadhesion agents as required.

6 Ensure that all the plant and equipment of the unit is ingood working order.

7 Instruct the construction team of the details of the workprogramme.

8 Inform the police and other organisations likely to beaffected by the surface dressing operation.

9 Inform the Materials Laboratory and arrange for thetesting of aggregates and binder and rate of spreadchecks during spraying.

The surface dressing operation7.2 The following sequence of events normallycomprises the complete surface dressing operation:

1 Raising the temperature of the binder in the depot tanksis started early in the morning so that the distributor canbe loaded with bitumen. The temperature should,preferably, be just above the ideal spraying temperature.

2 The supervisor arrives on site with traffic controlequipment and supervises the placing of warning signs,control barriers, traffic cones, etc.

3 The surface dressing unit arrives on site and thedistributor is parked off the road, preferably on a levelsite where the tank can be 'dipped' before and afterspraying. If the parking area is not level and alternativesite must be located.

4 Whilst the binder temperature is adjusted using theburners; binder is circulated through the spray bar, andthe jets are checked for correct operation.

5 The calibrated 'dip-stick' supplied with the distributor isused to measure the volume of binder in the tank at thestart of the day's work.

6 The supervisor instructs the distributor crew on thespray rate required, the corresponding road speed andthe pump output, where this is necessary.

7 The chipping crew load the tipper lorries with chippingsand the lorries line up ready to follow the distributor atthe location specified by the supervisor. The rollers alsoprepare to follow the distributor after the chippinglorries.

8 The road is thoroughly swept and road furniture such asmanhole covers, reflective studs etc, is masked so as toprevent contamination with binder.

9 Cut-off sheets of paper or other material are placed atthe beginning and end of the spray run. The supervisorchecks that the road is in fit condition for spraying andthat laboratory staff, if present, are ready to do traytests.

10 The burners on the distributor are extinguished and thedistributor is positioned at the beginning of the sprayrun.

11 The driver adjusts the guide chain, the fifth wheel islowered to the ground, and the height of the spray bar isadjusted.

12 The distributor then commences the spraying run, thecut-off sheets being removed immediately thedistributor passes to avoid contamination of the wheelsof the chip spreader or tipper lorries.

13 The chip spreader, tippers and the rollers should followclosely behind the distributor. Spraying should hestopped if the chipping operation is delayed for anyreason. A strip of binder 150mm wide is left un-chippedat the edge of the lane to allow for the overlap of theadjacent run of the distributor.

14 A tipper and crew should move slowly over the newdressing, spreading chippings by hand shovels on areaswhere there is a deficiency of chippings.

15 The operation is then repeated on the adjacent pass (ifany) and traffic is allowed to move slowly over the newdressing.

16 The distributor then returns to the original level parkingsite and the volume of binder remaining in the tank ischecked with the 'dip-stick'. The supervisor records theamount of binder used and, knowing the total areasprayed, calculates the average rate-of-spread.

17 Speed control and other traffic warning signs are left inposition along the length of the new surface dressing.

18 At the completion of the day's work the distributorspray bar is cleaned, all vehicles and plant refuelled andlubricated and the supervisor checks that the bitumenheaters are loaded ready to supply the binder requiredfor the next day.

After-care7.3 After-care is an essential part of the surfacing processand consists of removing excess chippings within 24 to 48hours of the construction of a dressing. Some of the excesschippings will have been thrown clear by passing vehicles butsome loose chippings will remain on the surface and these area hazard to windscreens and, hence, a source of publiccomplaint. They can be removed by brooming or bypurposed-made suction cleaners. Care must be taken withbrooming to avoid damage to the new dressing and it isusually best to do this work in the early

23

morning when the surface dressing binder is still relativelystiff. It is Important to stress that over-chipping can reducethe quality of a dressing, make after-care a more timeconsuming process and also unnecessarily increase costs.

8 Other surface treatments

8.1 Apart from surface dressing there are several otherkinds of surface treatment that complement surfacedressings, five of which are described briefly below.

Slurry seals

8.2 A slurry seal is a mixture of fine aggregates, Portlandcement filler, bitumen emulsion and additional water(ASTM, D 3910, 1996; BS 434, Parts 1 and 2, 1984).When freshly mixed they have a thick creamy consistencyand can be spread to a thickness of 5 to 10 mm. Thismethod of surfacing is not normally used for newconstruction because it is more expensive than surfacedressing, does not provide as good a surface texture, and isnot as durable as a properly designed and constructedsurface dressing. Slurry mixes are best made and spread bypurpose made machines as shown in Figure 8.

8.3 Slurry seals are often used in combination with asurface dressing to make a 'Cape-seal'. In this technique theslurry seal is applied on top of a single surface dressing toproduce a surface texture which is less harsh than a surfacedressing alone and a surface which is flexible and durable.However, the combination is moreexpensive than a double surface dressing and requirescareful control during construction.

8.4 Both anionic and cationic emulsions may be used inslurry seals but cationic emulsion is normally used inslurries containing acidic aggregates, and its'

early breaking characteristics are also advantageous whenrainfall is likely to occur. Suitable specifications for slurryseals and for a Cape-seal are given in Tables 10 and 11.

Table 10 Aggregate particle size distribution for slurryseals

Percentage by mass of totalaggregate passing test sieve

BS test sieve (mm) Fine General Coarse

10 - 100 1005.0 100 90-100 70-902.36 90-100 65-90 45-701.18 65-90 45-70 28-500.6 40-60 30-50 19-340.3 25-42 18-30 12-250.15 15-30 10-21 7-180.075 10-20 5-15 5-15

Bitumen content(per cent by massof dry aggregate) 10-16 7.5-13.5 6.5-12.0

The optimum mix design for the aggregate, filler, water andemulsion mixture should be determined using ASTMD 3910-84 (1996).

Table 11 Typical coverage for a new 'Cape seal'

Size of chipping in surfacedressing (mm) Coverage (m2 /m2)

20 130-17014 170-24010 180-250

Figure 8 Slurry seal machine (diagrammatic)

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Otta seal8.5 An Otta seal is different to surface dressing in that agraded gravel or crushed aggregate containing all sizes,including filler, is used instead of single sized-chippings.There is no formal design procedure but recommendationsbased on case studies have been published (NorwegianPublic Roads Administration, 1999). An Otta seal may beapplied in a single or double layer. Evidence on theperformance of these types of seal has shown them to havebeen satisfactory for over 12 years on roads carrying up to300 vehicles per day (Overby, 1998).

8.6 The grading of the material is based on the level oftraffic expected. Recommended grading envelopes aregiven in Table 12. Generally for roads carrying light traffic(<100 vehicles per day), a 'coarse' grading should bechosen while a 'dense' grading should be applied to onecarrying greater than 100 vehicles per day.

Table 12 Otta seal aggregate grading requirements

Percentage passing¹

Sieve (mm) Dense Coarse

19.0 100 10016.0 79-100 77-10012.0 61-100 59-1009.5 42-100 40-854.750 19-68 17-462.360 8-51 1-201.180 6-40 0-100.600 3-30 0-30.300 2-21 0-20.150 1-16 0-10.075 0-10 0-1

¹ Aggregate should be screened to remove stone greater than 19mm

8.7 The viscosities of binders used in construction shouldreflect the quality of aggregate employed but normally cutback bitumen MC 800, MC 3000 or 150/200 penetrationgrade bitumen is used depending upon the traffic volumesand type of aggregate cover. Spray rates can not becalculated by design and must be chosen empirically.Typically, spray rates (hot) for single seals are between 1.6and 2 1/rn 2 but reference must be made to the Design Guide(NPPA, 1999) so that necessary detailed adjustments canbe made.

8.8 It is because of the broad range of materials that maybe used and the empirical nature of the design of this typeof seal that it is imperative that pre-construction trials becarried out. This strategy will identify any special localconditions concerning the available aggregates and bindersto become apparent to enable the engineer to adjust thenominal design.

8.9 An important aspect of Otta seal construction is theneed for extensive rolling by pneumatic rollers for two orthree days after construction. The action of rolling ensures thebinder is forced upwards, coating the aggregate, and therebyinitiating the process, continued by subsequent trafficking, offorming a premix like appearance to the surface.

8.10 After care can take as long as twelve days andinvolves sweeping dislodged aggregate back into thewheelpaths for further compaction by traffic.

Sand seals8.11 Where chippings for a surface dressing areunobtainable or are very costly to provide, sand can be usedas 'cover material' for a seal. Sand seals are less durable thansurface dressings; the surface tends to abrade away undertraffic. Nevertheless a sand seal can provide a satisfactorysurfacing for lightly trafficked roads carrying less than 100vehicles per lane per day.

8.12 It is not possible to design a sand seal in the samesense that a surface dressing can be designed. The particles ofsand become submerged in the binder film, and the net resultis a thin layer of sand-binder mixture adhering to the roadsurface.

8.13 The sand should be a clean coarse sand, with amaximum size of 6mm, containing no more than 15 per centof material finer than 0.3 mm and a maximum of 2 per cent ofmaterial finer than 0.1 5mm. The sand should be applied at arate of 6 to 7 x 10-³ m3 /m2 (CSRA, 1986). The binder, whichmay be a cutback or an emulsion, should be spread at a rate ofapproximately 1.0 to 1.2 kg/m2 depending on the type ofsurface being sealed.

Synthetic aggregate and resin treatments8.14 These treatments are costly and are used only onrelatively small areas, usually in urban situations. where highskidding resistance is required. The aggregate is normally asmall. single-sized, calcined bauxite which has a highresistance to polishing under traffic. The aggregate is held bya film of epoxy-resin binder (Denning, 1978). The processrequires special mixing and laying equipment and is normallyundertaken by specialist contractors.

Applications of light bitumen sprays8.15 There are two main uses for light sprays of bitumen:

• A light film of binder which can be applied as the finalspray on a new surface dressing. The advantage of thisprocedure is that the risk of whip-off of chippings underfast traffic is reduced. This is particularly useful wheremanagement of traffic speed is difficult.

25

• A light spray of binder can be used to extend the lifeof a bituminous surfacing. This is particularly usefulwhere a surfacing is showing signs of bitumen ageingby fretting or cracking.

8.16 These applications may be referred to by differentauthorities as Fog Sprays or Enrichment sprays.

Fog sprays8.17 A light spray of bitumen emulsion is ideal forimproving early retention of chippings in a new dressing(CSRA, 1972). The road surface is usually dampenedbefore spraying or, if a low bitumen content emulsion (45per cent) is available,. this dampening can be omitted.Complete breaking of the emulsion must occur beforetraffic is allowed onto the dressing and it may be necessaryto dust the surface with sand or crusher fines to preventpick-up by traffic. If emulsion is diluted with water, toobtain a 45 per cent bitumen content to ensure the bitumenwill flow around the chippings, then the suitability of thewater must be established by mixing small triai batches.

8.18 The spray rate for the diluted emulsion will dependupon the surface texture of the new dressing but the bestresults will be achieved if the residual bitumen in the fogspray is treated as part of the design spray rate for thesurface dressing. The spray rate is likely to be between 0.4and 0.8 litre/m2. It is important to avoid over application ofbitumen which could result in poor skid resistance.

Enrichment sprays8.19 Surfaces which are showing obvious signs ofdisintegration through bitumen ageing can be enriched byapplying stable grade anionic bitumen emulsion which hasbeen diluted at a rate of 1: 1 with water (CSRA, 1972). Therate of application will depend upon the texture of thesurfacing and this must be determined by trial sprays,however, it is likely to be between 0.2 and 0.5 litres/m2 ofresidual bitumen. Great care must be taken to avoid leavinga slippery surface and a light application of sand sized finesmay be require in some cases.

9 References

Asphalt Institute (1983). Specifications for paving andindustrial asphalts. Specification Series No.2 (SS-2).College Park, Maryland (The Asphalt Institute).

American Society for Testing and Materials(1998). D2097, Standard specification for cutbackasphalt (Medium Curing Type). Annual Book ofASTM Standards, Vol.4.03. Philadelphia.

26

American Society for Testing and Materials (1996).D3910, Annual book of ASTM standards, Vol.4.03.Philadelphia.

British Standards Institution (1972). Cold emulsionspraying machines for roads. Part 2, Metric Units. BritishStandard, BS 3136:Part 2:1972. (British Standards Institution,London).

British Standards Institution (1984). Specification forbitumen road emulsions (anionic and cationic). BritishStandard 434:1984 (Parts 1 and 2). (British StandardsInstitution, London).

British Standards Institution (1985). Testing aggregates;sieve tests. British Standard, BS 812: Part 103.1:1985. (BritishStandards Institution, London).

British Standards Institution (1986). Specification for testsieves. British Standard 410:1976. (British StandardsInstitution, London).

British Standards Institution (1987). Specification forsingle sized aggregates for surface dressing. British Standard,BS 63: Part 2:1987. (British Standards Institution, London).

British Standards Institution (1989). Specification for hotbinder distributors for road surface dressing. British Standard1707:1989. (British Standards Institution, London).

British Standards Institution (1989a). Testing aggregates:method for sampling. British Standard, BS 812: Part102:1989. (British Standards Institution, London).

British Standards Institution (1989b). Testing aggregates:method for determination of the polished-stone value. BritishStandard, BS 812: Part 114:1989. (British StandardsInstitution, London).

British Standards Institution (1990a). Testing aggregates:method for determination of aggregate crushing value (ACV).British Standard, BS 812: Part 110:1990. (British StandardsInstitution, London).

British Standards Institution (1990b). Testing aggregates:method for determination of aggregate abrasion value (AAV).British Standard, BS 812: Part 113:1990. (British StandardsInstitution, London).

Committee for State Road Authorities (1972).TRH 7. The use of bitumen emulsions in the constriction andmaintenance of roads. Technical Recommendations forHighways. Department of Transport, Pretoria.

Committee for State Road Authorities (1986).TRH 3. Surfacing seals for rural and urban roads andcompendium of design methods for surfacing seals used in theRepublic of South Africa. Technical Recommendations forHighways. Department of Transport, Pretoria.

Denning J H (1978). Epoxy- resin/calcined bauxite surfacedressing on A1, Sandy, Bedfordshire: skid resistancemeasurements 1968 to 1977. Laboratory Report LR867.TRL Limited, Crowthorne.

Dickinson E J (1984). Bituminous roads inAustralia. Australian Road Research Board, VermontSouth, Victoria.

Hanson F M (1934). Bituminous surface treatmentof rural highways. Proc. New Zealand Soc. Civ.Engrs., Vol 21.1934/35 (New Zealand Society ofCivil Engineers).

Hitch L S (1981). Surface dressing in developingcountries: research in Kenya. Laboratory ReportLR 1019. TRL Limited, Crowthorne.

Hitch L S and Stewart M (1987). The preparationof cutback bitumens to ASTM specifications byblending readily available constituents. ResearchReport RR 104. TRL Limited, Crowthorne.

Jackson G P (1963). Surface dressing. ShellInternational Petroleum Co., London.

National Association of Australian State RoadAuthorities (1986). Principles and practice ofbituminous surfacing Vol 1: Sprayed work. (NationalAssociation of Australian State Road Authorities,Sydney).

Norwegian Public Roads Administration (1999). Aguide to the use of Otta Seals. Publication No.93.Directorate of Public Roads, Road TechnologyDepartment, International Division. Oslo.

Overby C (1998). Otta seal - a durable and costeffective global solution for low volume sealed roads.9th REAAA Conference, 'An International Focus onRoads: Strategies for the Future'. Wellington.

Salt G F and Szatkowski W S (1973). A guide tolevels of skidding resistance for roads. Laboratory ReportLR510. TRL Limited, Crowthorne.

Transport Research Laboratory (1992). Designguide for road surface dressings. Road Note 39 3rdedition. (TRL Limited, Crowthorne).

Transport Research Laboratory (1996). Designguide for road surface dressings. Road Note 39 4thedition. (TRL Limited, Crowthorne).

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Appendix A: Requirements for grading and particle shape (Reproduced fromBS 63: Part 2: 1987)

Specified sizes for given nominal sizes are given in Table Al and the grading limits for each nominal size of aggregate are givenin Table A2. Specified values which can be applied to roads carrying up to 250 vehicles per day are given in Table A3.

Table Al Specified sizes for given nominal sizes (mm)

Specified size

Nominal Passing Retained onsize BS test sieve 1 BS test sieve

20 20 14

14 14 10

10 10 6.3

6 6.3 3.35

1 In accordance with BS410 (1986), specification for testsieves

Table A2 Grading limits, specified size and maximumflakiness index for surface dressing aggregates

Nominal size of aggregates (mm)Grading limitsBS test sieve1 20 14 10 6.3

28 100 - - -20 85-100 100 - -14 0-35 85-100 100 -10 0-7 0-35 85-100 1006.3 - 0-7 0-35 85-1005.0 - - 0-10 -3.35 - - - 0-352.36 0-2 0-2 0-2 0-100.600 - - - 0-20.075 0-1 0-1 0-1 0-1

Minimum percentage by massretained on BS test sieve

Specified sizeSpecified size 65 65 65 65

Maximum flakiness indexMaximumflakinessindex 25 25 25 -

1 In accordance with BS 410 (1986), Specification for testsieves

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Table A3 Grading limits, specified size and maximumflakiness index for surface dressingaggregates for lightly trafficked roads

Nominal size of aggregates (mm)Grading limitsBS test sieve' 20 14 10 6.3

28 100 - - -20 85-100 100 - -14 0-40 85-100 100 -10 0-7 0-40 85-100 1006.3 - 0-7 0-35 85-1005.0 - - 0-10 -3.35 - - - 0-352.36 0-3 0-3 0-3 0-100.600 0-2 0-2 0-2 0-20.075 - - - -

Minimum percentage by massretained on BS test sieve

Specified sizeSpecified size 60 60 65 65

Maximum flakiness indexMaximumflakinessindex 35 35 35 -

1 In accordance with BS 410 (1986), specification for testsieves

Appendix B: Recommended polished stone values of chippings for roads inBritain

Table B1 Minimum PSV of chippings for roads in Britain

Reference: Design Manual for Roads and Bridges. HD 28/94 (DMRB 7.3.1) Stationery Office, London.

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Appendix C: The immersion tray test for determining the concentration ofadhesion agent required

The following test procedure has been has been included inRoad Note 14 (1964) and editions of Road Note 39, DesignGuide for Road Surface Dressing (in the UK) since at least1964 and it has in the past been published unchanged inORN3. The method is reproduced below and then suggestionsare made which may help to make it more appropriate fortropical conditions.

In this test a tin lid approximately 135mm diameter is coveredwith 15 to 20g of binder giving a film some l.5mm thick.When this has cooled to the test temperature* it is immersedin water also at test temperature to a depth of about 25mm.Nominal l4mm chippings are then applied by hand and lightlypressed in. At least six pieces of the aggregate are used. Thechippings are left for 10 minutes and are then carefullyremoved from the binder film: the percentage of binderretained on the chippings is assessed visually.

When testing an adhesion agent, a known quantity of agent isadded to the binder and thoroughly stirred to ensure gooddispersion. The procedure is then as outlined above. The testis repeated with varying concentrations of agent in the binderuntil the minimum concentration required to give satisfactoryresults has been found. The concentration normally falls in therange 0.5 to 2.5 per cent by mass of agent.

The agent may be considered satisfactory for use on the roadif, in the test, when the chippings are lifted from the binderfilm the faces which have been in contact with the film are all90-100 per cent coated with binder.

* The temperature of water and tray of binder in theabove test should be the expected temperature of the roadsurface during the treatment. Where it is desired tocompare the behaviour of different agents with a givenstone and binder it is suggested that 200C should be usedas the test temperature.

Suggested new procedureFor tropical conditions the test bitumen should be of the gradeto be used on site and it should be tested at appropriate sitetemperatures. Testing different adhesion agents at 200C is notpractical if, for instance, hot conditions warrant the use of an80/100 penetration grade bitumen. It is considered appropriateto test the adhesion agents at a temperature which relates tothe design road temperature on which binder selection wasbased.

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A tin lid approximately 135mm diameter, or other suitabletray, is covered with binder to give a film some l.5mm thick.Place at least 10 chippings which are damp, but not with shinywet surfaces, in the film of binder at the 'design roadtemperature' and leave for 10 minutes. Then withdraw someof the chips to confirm coating. Add water to about half thedepth of the remaining chippings at the chosen testtemperature and leave for 10 minutes before withdrawingthem and noting the degree of coating. If the coating is lessthan 90 per cent on any chipping then an adhesion agentsshould be tried. In this case different percentages of theadhesion agent are added to samples of the binder until 90-100 per cent coverage is obtained, after soaking, on allchippings.

If limestone chippings are available they will provide a goodcomparison of adhesion properties with the chippings to beused on site because limestone has good affinity with bitumen

Appendix D: The probe penetration for test for measuring road surfacehardness

General descriptionThis test utilises a modified soil assessment conepenetrometer, originally designed by the UK MilitaryEngineering Experimental Establishment for the assessmentof in-situ Soil strength. The standard cone normally used withthis penetrometer is replaced by a 4mm diameter probe rodwith a hemispherical tip made of hardened steel. The probe isforced into the road surface under a load of 35 kgf (343N)applied for 10 seconds and the depth of penetration ismeasured by a spring loaded collar that slides up the proberod. The distance the collar has moved is measured with amodified dial gauge. The temperature of the road surface isrecorded and a graphical method is used to correct the probemeasurements to an equivalent value at a standardtemperature of 300C.

Method of operationAll measurements are made in the nearside wheel track ofeach traffic lane where maximum embedment of chippingscan be expected. A minimum of ten measurements arerequired at each location. These should be evenly spacedalong the road at intervals of 0.5m. any recently repaired orpatched areas being ignored. For convenience themeasurement points can be marked with a chalk

cross. The probe tip should not be centred on any large stonespresent in the road surface.

Before each measurement the collar is slid down the proberod until it is flush with the end of the probe. The probe isthen centred on the measurement mark and a pressure of 35kgf is applied for 10 seconds care being taken to keep theprobe vertical. The probe is then lifted clear and the distancethe collar has slid up the probe is recorded in millimetres.

It sometimes occurs that the point selected for test is belowthe general level of the surrounding road surface. It is thennecessary to deduct the measurement of the initial projectionof the probe tip from the final figure.

The road surface temperature should be measured at the sametime that the probe is used and the tests should not be madewhen the surface temperature exceeds 350C. This will limitprobe testing to the early morning in many tropical countries.The probe readings are corrected to a standard temperature of300C using Figure D 1, and the mean of ten probemeasurements is calculated and reported as the meanpenetration at 300C. Categories of road surface hardness andthe corresponding ranges of surface penetration values areshown in Table 4 (paragraph 5.5).

Figure D1 Graphical method for correcting measurements of road surface hardness to the standard test temperature of 30oC

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Appendix E: Example of a surface dressing design

Site description Aggregate (nominal 19 mm)A two-lane trunk road at an altitude of approximately Median size (ie 50 per cent passing) l6mm

15OOm. Flakiness Index 16Average Least Dimension 12

Vehicle count averaged 3370 per day/lane (i.e. (from nomograph, Figure 6)'Heavy' rating).

Bitumen to be used is 400 penetration grade (made The determination of spread rates of 80/100 and 400by cutting back 80/100 pen bitumen with 6.7 per pen bitumen for an F factor of -5 and an ALD of 12cent by mass (or approximately 7.5 per cent by on a site where maximum durability is required arevolume) of a 3:1 mixture of kerosene and diesel. summarised in Table El.

Design Factor

Traffic (Heavy) -3Existing surface (average bituminous) -1Chippings (cubical) 0Climate (hot/dry) -1Overall weighing factor (F) -5

Table El Determination of spread rates for 400 penetration grade bitumen

Spread rates for penetrationBasic spread rate For increased grade binders (kg/m2)

R for MC3000 durability(from Fig. 7 or equation 1) RD = (R*1.08) 80/100 pen 400 pen

Type Terrain (kg/m2) (kg/m2) (RD *0.9) (RD 0.9 1.067)

Flat 0.89 0.96 0.87 0.92Uphill grade>3% 0.89*0.9=0.80 0.87 0.78 0.84Downhill grade>3% 0.89*1.1=0.98 1.06 0.95 1.02

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Appendix F: Tests for uniformity of transverse distribution of binder (depottray tests) (reproduced from British Standard 1707:1989)

GeneralThis standard includes requirements and tolerances foruniformity of distribution of binder across the surface beingsprayed. A standard method for determining the transverseuniformity of distribution has been developed, therequirements of which are:

• The conditions prevailing during the test are comparablewith those occurring during normal operations asregards:

a Temperature of binder.

b Viscosity of binder.

c Height of distributor gear above the test surface.

d Pressure in the distribution system.

e Speed of operation of mechanical distributing gear,when applicable.

• The test surface is divided into strips of equal width,usually 50 mm, the length of the strips being parallel tothe direction of travel of the distributor.

• The test is so arranged that the distributor can operate fora sufficient period to obtain the normal workingconditions, and when this has been achieved the testsurface is exposed to the discharge for a suitable period.

• The amount of binder delivered on each 50 mmstrip is then measured and the results expressed as apercentage deviation from the mean for all the 50mmunits over the effective width. (The effective width isdefined as the sprayed width less the 150 mm margin ateach side.)

•• The rests of the test are recorded in the form indicated inFigure Fl.

Depot tray test apparatus and procedureThe apparatus consists of a wheeled trolley carrying a set ofremovable containers. Each container is 50 mm wide x 1000mm long x 150 nun deep, made of 0.9 mm mild steel sheet,and of approximately 7 litres capacity. The containers extendto a width of 150mm greater than the full spray width of thedistributor there being six containers per 300 mm of spraywidth. The rim of each container is lipped on one side in orderthat the containers will overlap and prevent binder escaping.

Before each test the containers are examined for damage andreplacements made if such damage is likely to affect the test.The trolley runs on steel rails fastened to the top of a 1500litres catch tank, the rails being horizontal and parallel to thesides of the tank and sufficiently long to allow the trolley tolie clear of the spray before the test. The top rim of eachcontainer, when fitted on the trolley is parallel to the rails andthe same distance below the nozzles or distributing gear as theroad surface under normal working conditions.

The distributor is backed into position with the spray bar overthe catch tank, precautions being taken to see that the spray-bar is horizontal and at right angles to the rails. The trolleyand containers rest on the rails clear of the spray hood. Ashort preliminary spray is made to ensure that all nozzles arefunctioning and that the machine is otherwise in normalworking condition.

The trolley and containers are then pushed underneath thespray hood; spraying is commenced, and maintained for aperiod of time sufficient almost to fill the containers. Thetrolley is then withdrawn to the previous position.

The depth of binder in each container is measured by dippingwith a steel rule graduated in millimetres. Each container isdipped in the same position, a convenient place being some300 mm from one end. Dipping takes place when the froth hassettled.

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Figure F1 Typical results for uniformity of transverse distribution of binder. Reproduced from British Standard 1707:1970

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Appendix G: Bitumen distributor: rate of spread/speed calibration

The output of the spray bar will have beendetermined for the given bitumen, operatingtemperature and distributor settings. In the case ofconstant pressure distributors, the operating pressuremust be fixed . Constant volume distributors requireseparate calibrations for different combinations ofbitumen delivery pump speed and number of jets inoperation. By varying one of these variable at a timecalibration charts can be developed.

Having completed the static calibration tests, so thatthe rate of delivery of bitumen is known, anddetermined the design spray rate the required roadspeed of the distributor can be calculated fromequation G1.

Speed of distributor = S/(R*W) (G1)

Where S is the rate of delivery (mass) from the

spray bar in kg/s

R is the design spray rate (mass) in kg/m2

W is the sprayed width in metres

The required speed will then be in m/s. Thecalibration should then be checked dynamically asconstruction work progresses as described in‘Method B’' in paragraph 6.29.

Appendix H: Miscellaneous equipment required for a surface dressing unit

1 Fuel and lubricant for servicing plant. (apurpose-built vehicle is often used and this is veryeffective).

2 Temporary traffic warning signs.

3 Stop/go signs and warning flags.

4 Hand brooms, shovels, wheelbarrows (forchipping small areas) watering cans, hammers andchisels (for opening drums etc.).

5 Masking tape for protecting road furniture and coveringroad markings.

6 Sample cans and bags.

7 Cut-off sheets of building paper or flattened drums forclean starting and finishing of spraying.

8 ‘Rotatherm' type thermometers.

9 Cleaning materials for plant and personnel.

10 A walking type distance measuring device.

11 First aid kit, including burn treatment supplies andsupply of water

12 Fire extinguishers.

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